This invention relates to medical stents. Medical stents are well know for use in opening and reinforcing the interior wall of blood vessels and other body conduits.
Stents are generally tubular in configuration and radially expandable. They may be of the self-expanding type or they may be mechanically expandable with an outward pressure applied to the stent. Typically, the later type stents are expanded by means of an interiorly positioned balloon. Stents may be made of various materials such as plastic or metal. Presently, metal is the material of choice.
This invention is specifically concerned with stents in the form of a closed cylinder which is made up of a plurality of struts, the struts being deformed either permanently or otherwise upon expansion of the stent.
To-date, stents, particularly coronary stents, have been made up of elements that are relatively large and of the following order:
These dimensions and all other dimensions referred to herein after refer to the stent in its expanded state.
The basic idea behind the present invention is to provide a stent of fine structure (micro structure) that provides adequate vessel support but the openings therein are so small that the stent creates minimal disruption of the vessel surface and is so fine that it is for all practical purposes xe2x80x9cinvisiblexe2x80x9d to the body in which it is implanted and to the body constituents such as blood flow.
An analogous example is a window screen, the idea being to provide a screen (stent) to support a vessel but which is from the stand point of the various physiological aspects of the body so fine as to be effectively xe2x80x9cinvisiblexe2x80x9d and for all practical purposes can then be said to be considered by the body as being nonexistent.
There does exist in the art one example of an ultra thin micro porous nickel titanium foil which is rolled in the fashion of a jelly roll to provide a self-expanding stent. Self-expansion is provided by the natural unrolling tendency of the tightly wound stent following its implantation. However, this type of stent has not been widely acceptable and differs from the stents of the present invention in that no strut deformation occurs with respect to the elements making up the foil or screen of the ultra thin micro porous jelly roll type stent. Cardiovascular Dynamics, Inc., has published material concerning the xe2x80x9cjelly rollxe2x80x9d stent.
In contrast to the above-identified prior art, this invention provides stents of closed cylindrical construction in which expansion is accompanied by deformation of the strut structure making up the body of the stent. As already pointed out, the term deformation is meant to include other deformation in addition to permanent deformation as used in the metallurgical sense. In accordance with this invention a suitable micro structure design can be obtained by dimensionally constructing a stent having a reduction ratio as compared to current coronary stents of 4:1 to 10:1. Note with reference to ratio reductionxe2x80x94current dimensions are thereby reduced by a factor of about 4-10.
Even more specifically, micro structure stents in accordance with the present invention will preferably have about the following dimensions:
(current stent designs typically have a maximum PIN opening of around 0.025 inches to 0.050 inches in diameter).
The term xe2x80x9cmaximum PIN openingxe2x80x9d is used herein to describe micro openings in which the dimensions specify the largest which can be passed through the cell opening. This applies as noted above to the expanded stent configuration. Typically, as a stent is expanded to larger diameters, the opening becomes larger. It is believed that using a maximum PIN opening specification that the concept of the present invention may be more readily applicable to stents of either open or closed cell geometries.
The preferred stents having a micro structure in accordance with the present invention will also have a wall thickness of up to about 0.004 inches, which is not required according to the invention, but provides adequate radiopacity and handling characteristics in the stent structure.
In addition to directly forming a micro structure in the wall of a stent, this invention may be accomplished by providing a stent within a stent wherein both stents, even though having larger openings than would be characterized as providing micro structure, may in fact provide micro structure by control of the registration of the openings in the stents as is more fully described below.
The stent within a stent combination leads to other embodiments of the invention which are also described more fully below.
As with any stent design, many different designs are possible with respect to features such as flexibility, etc. The geometries which are shown hereinbelow are included only for illustrative purposes.